Process for recovering beryllium sulfate



Patented Get. 8, 1946 PRGCESS FOR RECOVERING EERYLLIUM SULFATE Bengt R.F. Kjellgren, University Heights, Ohio, assignor to The Brush BerylliumCompany, Cleveland, Qhio, a corporation of Ohio Application April 2,1942, Serial No. 437,337 In Canada July 3, 1941 1 Claim. 1

This invention relates to a process for the production of berylliumsulfate and has for its object the production of beryllium sulfatesubstantially free of contamination with calcium, this application beinga continuation in part of my earlier application Serial No. 350,981,filed August 3, 1940.

In the production of beryllium sulfate by methods employed prior to thepresent invention it has been found difiicult in commercial operation toproduce beryllium sulfate free or nearly free from contamination withcalcium. For certain uses of beryllium sulfate substantial freedom fromsuch contamination is required and an important need arose for apractical method capabio of uniformly producing beryllium sulfate thatis substantially calcium-free. For example, an important use ofberyllium sulfate has been involved in the production of berylliumoxide, the latter being produced by first preparing beryllium sulfateand then converting the sulfate to the oxide. Since calcium, in the formof calcium oxide, is a harmful and troublesome contaminant of berylliumoxide in the case of various uses of the latter, it became desirable toproduce the calcium-free sulfate in order to secure the uncontaminatedoxide.

The above noted difflculty in the production of calcium-free berylliumsulfate was encountered in the use of the method of preparing thesulfate disclosed in the United States patent to Sawyer and Kjellgren,No, 2,018,473. The erratic character of the results secured, withrespect to calcium contamination, were puzzling and unexplainable by anyknown properties or behavior of the substances involved. As a result ofextensive studies and experimental investigations carried out under mdirection it was discovered that calcium sulfate, as to solubility,presents a peculiar and, previous to the present invention,unpredictable behavior in solution with beryllium and ammonium sulfates.It was further discovered that the solubility of calcium sulfate in asaturated solution of beryllium sulfate is critically affected by thepresence in the solution of ammonium sulfate if the concentration of thelatter salt lies within certain limits hereinafter point-ed out.

present invention is based upon the above stated discoveries and bymeans of it the concurrent crystallization of calcium sulfate withberyllium sulfate can be prevented entirely or the crystallization ofthe calcium sulfate can be delayed in relation to the crystallization ofthe beryllium sulfate, to the end in either case that beryllium sulfatecan be crystallized and removed from contact with the contaminatedmother liquor prior to any appreciable crystallization of calciumsulfate and resultant contamination of the beryllium sulfate.

The behavior of calcium sulfate in a solution containing beryllium andammonium sulfates, as determined by the experimental investigationsreferred to above, is indicated by the accompanying drawing which showsgraphically the influence of free ammonium sulfate on the equilibriumsolubility of calcium sulfate (C5804) at room temperature, or about 20C., in a solution which contains approximately 635 grams of berylliumsulfate (BeSO4.4HzO) per liter and which therefore is approximatelysaturated with the latter salt. The expression free ammonium sulfate asused in the last preceding sentence and elsewhere in this application,means ammonium sulfate present in a solution of beryllium sulfate inaddition to the stoichiometric amount of ammonium sulfate needed toconvert to ammonium alum an aluminum sulfate that may have beenintroduced or included in th solution in its preparation, as, forexample, in the preparation from raw material such as beryl orecontaining a substantial amount of aluminum. The relationships shown inthe drawing have been determined experimentall by leaching an excess offreshly precipitated calcium sulfate with saturated beryllium sulfatesolutions containing different amounts of ammonium sulfate untilequilibrium conditions have been attained at approximately 20 C.

It will be observed from the graph that as the free ammonium sulfateconcentration of the solution is gradually increased, the solubility ofcalcium sulfate decreases at first from an initial value of about 1.7grams per liter when the solution is free of ammonium sulfate to aminimum value of about .5 gram per liter at an ammonium sulfateconcentration of approximately grams per liter. As the free ammoniumsulfate concentration is increased beyond this latter value, thesolubility of calcium sulfate increases abruptly reaches a sub-maximumof about 1.8 grams per liter at an ammonium sulfate concentration ofapproximately 210 grams per liter. Further increase in the ammoniumsulfate concentration results first in a slight reduction of the calciumsulfate solubility, but still further increases again raise the saidsolubility, a maximum solubility of about 3.1 grams of calcium sulfateper liter occurring in the presence of approximately 400 grams per literof free ammonium sulfate.- As the ammonium sulfate concentration isincreased further, the solubility of calcium sulfate decreases andreaches zero at an ammonium sulfate concentration of about 500 grams perliter. It will be understood that the area under the curve represents afield of relationships in which calcium sulfate remains soluble. In thefield above and to the extreme right of the curve, calcium sulfate isinsoluble under equilibrium conditions.

By reason of m discovery of the indicated influence of free ammoniumsulfate concentration upon the solubility of calcium sulfate in asaturated beryllium sulfate solution, and by reason of my furtherdiscovery that the free ammonium sulfate concentration imparts asupersaturating effect which will be described more fully hereinafter,it becomes possible to retain the calcium salts in solution whileberyllium sulfate is being crystallized from the solution. In brief, theobjectives of the invention are accomplished by adjusting the ammoniumsulfate concentration of the solution to a value. determinable from thecurve of the drawing, that is effective in holding the calcium sulfatecontent in at least supersaturated solution until after the berylliumsulfate has been crystallized and the crystals have been separated fromcontact with the resulting calciurn-contaminated mother liquor.

For the purpose of detailed description and full explanation of theinvention it will be convenient to consider its application to themethod of producing beryllium sulfate disclosed in the Sawyer andKjellgren Patent No. 2,018,473 to which reference has already been made.In that patented method a suitable raw material, such as beryl ore, istreated to render it soluble in sulfuric acid, and then is furthertreated with such acid to convert some components thereof to sulfates.The sulfated ore is then leached with water to extract the solublesulfates. The solution so obtained, after filtering, may containsulfates of Various elements that were present in the ore, the principalones of which are the sulfates of beryllium and aluminum. Such asolution can be treated in either of two ways disclosed in the patent toeffect a selective separation of beryllium sulfate from aluminumsulfate. These two alternative procedures will now be described, inturn, as modified by the addition of the present invention.

In applying the invention to the first procedure, the filtered leachingsolution obtained from the sulfated beryl ore and containing bothaluminum and beryllium sulfates is at first adjusted to a sub-saturatedconcentration of beryllium sulfate such as to produce a mother liquorsaturated in beryllium sulfate after the ammonium alum is crystallized,and then an amount of ammonium sulfate is added sufficient to convertall of the aluminum sulfate to alum and, in addition, to establish inthe mother liquor remaining after the alum is crystallized, aconcentration of free ammonium sulfate of preferably about 90 to 125grams per liter. The solutions may then be treated in any manner capableof causing the crystallization of beryllium sulfate. The calculationsrequired to produce the stated concentration in the saturated berylliumsulfate mother liquor obtained after the alum has been crystallized aremade quite simple by expressing the free ammonium sulfate concentrationas a percentage of the beryllium sulfate concentration. To illustrate,the saturated mother liquor, as previously stated, contains 635 grams ofberyllium sulfate (BeSO4.4H2O) per liter, and it is desired that thefree ammonium sulfate concentration therein be 4 between and 125 gramsper liter. Accordingly, the free ammonium sulfate concentration is to be90/635ths to 125/635ths of the beryllium sulfate concentration, or 14.1%to 19.7% of the latter, in the saturated mother liquor. It will berecognized that the same percentage relationship must also exist in thesub-saturated solution prior to the crystallization of the alum. Hence,to determine how much free ammonium sulfate should be added prior tocrystallization, all that is necessary is to determine the berylliumsulfate concentraticn in the sub-saturated solution, and then add from14.1% to 19.7% thereof as free ammonium sulfate. As will be noted fromthe curve in the drawing, the latter concentrations of free ammoniumsulfate are sufficient to reduce the solubility of calcium sulfate toabout a minimum value. Since the leaching solution from the sulfatedberyl ore generally contains no free ammonium sulfate, one may determinefrom the curve that it is capable of dissolving as much as about 1.7grams of calcium sulfate per liter if the solution is saturated inberyllium sulfate. After the ammonium sulfate has been added, however,and the solubility of calcium sulfate has been reduced to a minimum, thecalcium sulfate in excess of the minimum solubility will tend tocrystallize from the solution along with the alum crystals. The excessso crystallized would correspond to about 1.2 grams per liter. After asufficient period of time has elapsed to permit these crystallizationsto be completed, the solution may be filtered to remove the alumcrystals and the precipitate of calcium sulfate. If such filtration isperformed carefull so that all of the rather fine precipitate of calciumsulfate is retained on the filter, the resulting filtrate will have hadits calcium sulfate content reduced to about onehalf gram per liter andwill also be substantially free of aluminum ions. The filtrate isthereupon in condition for further treatment to crystalliz berylliumsulfate selectively therefrom. Such crystallization may be performed byevaporating the aluminum-free filtrate under vacuum. I prefer, however,to perform it by first evaporatin the filtrate by means of heat, therebyconcentrating it to the point where the filtrate is substantiallysaturated in beryllium sulfate at or near the boiling point of thesolution. Evaporation to about half its original volume is adequate, butthe evaporation may be either greater or less. After such concentrationhas been effected, the solution is transferred from the evaporator to acrystallizing tank where the temperature of the solution may be reducedat a suitably controlled rate. Since the solubility of beryllium sulfatedecreases with temperature, cooling of the solution causes berylliumsulfate to crystallize from the solution. After the solution has beencooled to about room temperature, the crystals of beryllium sulfate maybe removed from contact with the mother liquor by filtering orcentrifuging, or otherwise. Now if, for purposes of illustration. weassume that the aluminum-free filtrate is concentrated by evaporation tothe point where the concentration of beryllium sulfate in theconcentrated solution is about two times the concentration of berylliumsulfate in the aluminum-free filtrate, it willbe apparent that theammonium sulfate concentration and the calcium sulfate concentration inthe concentrated solution will have been increased in the sameproportion; that is. the ammonium sulfate concentration will have beenincreased from, say, grams per liter to 220 grams per liter, Likewise,the calcium sulfate concentration will have been increased from about0.5 gram per liter to about 1.0 gram per liter. Now, as the solution iscooled, and as beryllium sulfate crystallizes from it, the volume of theremaining mother liquor is gradually decreased so that theconcentrations of ammonium sulfate and calcium sulfate arecorrespondingly gradually increased. When the crystallization has beencompleted, with resultant removal from the solution of the correspondingter of crystallization, the over-all concentration ratio may have beenincreased from 2.0 (as it was after the evaporation step) to 2.8 afterthe crystallization step. In other words, the crystallization ofberyllium sulfate together with the slight effect of cooling hasincreased the ammonium sulfate concentration from 220 to 308 grams perliter, and of the calcium sulfate from 1.0 to 1.4 grams per liter.Referring now to the drawing it will be seen that these changes inconcentration can be plotted thereon so that the changes which havetaken place may be followed graphically. Thus point a represents theammonium sulfate and calcium sulfate concentrations in the cold,aluminum-free solution just prior to evaporation. Point b represents theconcentrations of these same sulfates in the hot solution afterevaporation, and point represents the concentrations in the cold motherliquor after the crystallization of beryllium sulfate has beencompleted. By joining these points with a line, it can be seen that theline represents the locus of all concentrations through which thesolution has passed while being treated in accordance with the procedurehere under discussion, Since the line joining the points lies whollywithin the solubility curve for calcium sulfate, it is apparent that atno time during the entire treatment has a condition prevailed whichwould allow calcium su fate to crystallize from the solution. The resultis that all the calcium sulfate has been held in solution during thetreatment, and the crystals of beryllium sulfate which have beenrecovered by selective crystallization are uncontaminated with calciumsulfate.

It will be understood from the preceding example that so far as theprinciples of the invention are concerned, there was no need to limitthe evaporation step to a concentration ratio of 2.0, since clearly thesolution could have been evaporated mor than this amount without havingapproached the limit of solubility of calcium sulfate therein. Thus itmight have been evaporated so to attain the concentrations repreby point0. The subsequent crystallization would have increased. theconcentrations along the line abc to some point such as d, for example.So long as point at falls Within the curve, no crystallization ofcalcium sulfate would occur. In brief, therefore, it will be seen thatthe limiting value of concentration is found by projecting line abc soas to intersect the curve at point e. So long as the concentrations ofammonium sulfate and calcium sulfate do not exceed the concentrationswhich are represented by point e, the calcium sulfate will remaindissolved in the mother liquor at room temperature. Ordinarily, suchhigh concentrations cannot be used practically, however, because thecrystal slurry becomes too thick to be handled conveniently in ordinaryequipment. Where such practical limitations do not arise, however, theprinciples of the invention may be relied on to accomplish the desiredresult of holding the calcium sulfate in solution.

It will-be recognized from the preceding example that an importantfeature of the preferred procedure is the step of establishing anammonium sulfate concentration in the aluminumfree solution of aboutgrams per liter. On the basis previously explained, this concentrationrepresents about 17.3% of the beryllium sulfate concentration. Theestablishment of this ammonium sulfate concentration not only effects areduction of the calcium sulfate concentration to about one-half gramper liter, but also determines the graphical path along which thesubsequent process steps conduct the solution. Thus it will be seen thatthe location of the point a, which is determined by the amount ofammonium sulfate present in the solution, in turn determines thedirection of the line abc with respect to the origin of the curve. Sinceall locus lines representing the path of solutions radiate about theorigin, it is apparent that if the point a were carelessly selected, theline abc might pass outside the solubility curve. Under such conditions,the benefits of the invention might be wholly lost. This possibility isdiscussed more fully hereinafter. It is advisable, therefore, toproportion the ammonium sulfate to the calcium sulfate content so thatthe latter salt will remain dissolved during all of the evaporation stepand the subsequent crystallization step. It will be noted that the lineabc represents a ratio between free ammonium sulfate and calcium sulfateof about 220 to 1. Such a ratio prevails if, by filtering the solutioncarefully, the calcium sulfate content in the filtrate has been reducedto gram per liter. Those skilled in the art will recognize, however,that sometimes it may not be feasible to perform the filtrationcarefully enough in commercial practice to remove all of theprecipitate. Then a problem arises in determining whether or not theinvention can be utilized for its intended purposes. For example, let usassume that the filtration is not performed carefully, and that part ofthe precipitate of calcium sulfate passes through the filter into thefiltrate. The concentration in the filtrate may then he, say .7 gram perliter instead of .5 gram per liter. The concentrations of the solutionthen be designated graphically by the point f, and it will be observedthat if a straight line is drawn through the origin of the curve so asto pass through point 1, it will, when extended, pass into a field ofconcentrations wherein calcium sulfate is insoluble under equilibriumconditions. Such field lies between points it and 7'. Between otherpoints on the line, such as between g and h, and between ;i and k, thecalcium sulfate is soluble. Consider, now, that the solution is treatedin accordance with the'first procedure as identifled above. Under suchtreatment, the concentrations would move from the point 1 to, say, thepoint in during evaporation. Interpreting the effects f such movement,it will be recognized that during the initial stages of the evaporationtreatment, the solution will contain undissolved crystals of calciumsulfate, but that after the concentrations have been increased to thevalues designated by the point g, these crystals will dissolve.Accordingly, when the concentrations designated by point as have beenreached, all the calcium sulfate will be in solution and the solutionshould be in readiness for the step of crystallizing beryllium sulfatetherefrom by cooling it. It will be appreciated from what has been saidpreviously, however, that as the crystallization proceeds, theconcentrations of ammonium sulfate and calcium sulfate are increased.Let us assume that these concentrations will reach the values designatedby the point 11. when the crystallization has been completed. Byreference to the curve, it will be seen that the point n lies in a fieldwhere the calcium sulfate would be insoluble if equilibrium conditionsprevailed. I have found, however, that under conditions such as areexemplified here, the calcium sulfate may not be precipitated as onewould expect by interpreting the curve. The explanation for this anomalyappears to be that such precipitation is prevented, at leasttemporarily, by reason of the fact that the solution undergoes acondition of supersaturation. In other words, the solution does notattain equilibrium conditions immediately upon passing out of the region971. into the region hi. On the contrary, it becomes supersaturated withrespect to calcium sulfate, and is sufficiently stable to remainsupersaturated for a rather extended period of time. Of course, if thedegree of supersaturation is carried to an extreme, then the solutionbecomes unstable and breaks down to precipitate calcium sulfate andultimately to reach equilibrium conditions. I have found, however, thatthe unstable condition of supersaturation may continue for many hours,and that as a result, the crystallization of beryllium sulfate may becarried out in such a solution without becoming contaminated withcalcium sulfate. It will be appreciated, however, that the crystalsshould be removed from contact with the supersaturated solution beforeit breaks down. Since the supersaturation may continue for as long asseven or eight hours, it is possible to effect such removal before thebreakdown occurs. The following example will illustrate this feature.

For the purpose of illustrating the supersaturating ffect which isintroduced by the presence of ammonium sulfate, a quantity ofaluminumfree solution containing about .7 gram of calcium sulfate perliter and about 110 grams of ammonium sulfate per liter was concentratedby evaporation to about double the above concentrations, as measured inthe hot concentrated solution. The hot solution was, at this time,substantially saturated in beryllium sulfate. It was next cooled at arelatively slow rate designed to permit crystallization of berylliumsulfate to continue for a period of about eight hours before roomtemperature would be reached. Samples were taken at twenty-minuteintervals during this period of time, and the samples so taken wereanalyzed for calcium. Samples of beryllium sulfate crystals taken duringthe first seven hours of the crystallization were found, upon analysis,to contain between .0005% and .002% calcium sulfate. The sample taken at7 hours and 20 minutes contained .05% calcium sulfate, and subsequentsamples taken at 7 hours and 40 minutes, eight hours, etc., contained asmuch as .07%. In view of these results, it will be apparent that thesolution broke down rather rapidly and permitted the calcium sulfate tocrystallize from the solution.

In view of the supersaturation effect which is encountered under theconditions just discussed, it becomes apparent that the insoluble fieldhi does not necessarily prevent one from using ammonium sulfate-calciumsulfate ratios which penetrate that field. In fact, I have found inpracticing the invention that ratios as low as 125 to 1, as designatedby line pq, may be employed satisfactorily. It should be recognized,however, that with such small ratios, the degree of supersaturation isapt to be more extreme than in the case of larger ratios so that thesolution is more apt to become unstable sooner. If one recognizes thisfact, however, he can usually shorten the crystallization periodsufficiently to enable him to separate the crystals of beryllium sulfatebefore the solution breaks down. Where this cannot be done conveniently,then these small ratios should be avoided. A ratio of 150 will usuallyaii'ord ample time for commercial working of the invention, and, ofcourse, if a ratio of 175 to 1 or more is used, the supersaturationeffect need not be relied on.

In view of the fact that a ratio as low as 125 to 1 is thoroughlyoperative for accomplishing the purposes of the invention, it will beapparent that a calcium sulfate concentration of about 0.9 gram perliter can be tolerated in the filtrate if an ammonium sulfateconcentration of about grams per liter also prevails in the filtrate.Accordingly, it is permissible to allow the filtration at point a to bedone in filtering apparatus which does not retain all of the precipitateof calcium sulfate. Nevertheless, it will be apparent that it isadvantageous to filter out as much as possible of the precipitate sinceif the calcium sulfate concentration in the filtrate is held to itsminimum value of gram per liter, a high ratio of ammonium sulfate tocalcium sulfate can be obtained with a minimum content of ammoniumsulfate.

It should be recognized that even though the filtration mentioned abovedoes not separate out all of the precipitate of calcium sulfate, it ispossible yet to avoid the uncertainties of operation which accompany areliance upon the supersaturation effect. For example, if afterfiltration the calcium sulfate concentration in the aluminum-freefiltrate is at, say, 0.9 gram per liter, it is not necessary that thesolution be treated so to move along line pq during the evaporation andcrystallization steps. Instead, further additions of ammonium sulfatemay be made to the filtrate before evaporation is started, the additionbeing of such amount as to increase the ammonium sulfate-calcium sulfateratio to any desired value. For example, by adding enough ammoniumsulfate to the filtrate to raise the ratio to 220 to 1, the evaporationand crystallization steps will proceed along line abc instead of alongline pq. Likewise, the addition may be such as to establish any otherdesired ratio which will not extend into a field where supersaturationmay be encountered. In general, however, ammonium sulfate concentrationsin the filtrate of greater than about 350 grams per liter should beavoided since such high concentrations are apt to lead to the practicaldifficulties mentioned above in connection with the handling of thickcrystal slurries, particularly after the filtrate has been evaporatedand crystallized. Consequently, where the calcium content in thefiltrate is as much as 0.9 gram per liter, ratios of over about 350 to 1should be avoided. Where the calcium content is around one-half gram perliter, the ratio in the filtrate may be as high as 700 to 1. It will beunderstood that in all events, the filtrate should be checked forammonium sulfate and calcium sulfate concentrations before evaporationis started, to determine that a ratio of at least to 1 exists. For thiscondition at least about 80 grams of ammonium sulfate per liter must bepresent when the solution is saturated in calcium sulfate, as willappear from the lowest intersection of the line pq with the solubilitycurve. Additions of ammonium sulfate should be made in case the ratio isfound to be less, and as pointed out, the additions can be of suchquantities as will establish any desired ratio greater than 125 to 1, solong as the ammonium sulfate concentration is maintained less than about350 grams per liter.

In applying the invention to the second procedure of the Sawyer andKjellgren Patent No.

2,018,473 referred to above, let us assume that the leaching solutioncontaining aluminum sulfate, beryllium sulfate and calcium sulfate istreated in accordance with the second procedure described in the patent.As there described, ammonium sulfate would be added to the leachingsolution to convert the aluminum sulfate to alum and to provide asuitable excess required to render the alum insoluble. This excess, orfree ammonium sulfate concentration might be, say, 60 grams per liter.If the leaching solution were saturated in calcium sulfate before theaddition of the ammonium sulfate, the addition would depress thesolubility and cause some calcium sulfate to crystallize out of thesolution. According to the curve, the calcium sulfate so crystallizedwould correspond to a decrease of about 0.7 gram of calcium sulfate perliter, and this amount would appear in the mixed crystals of alum andberyllium sulfate. Now the problem is to recover calcium-free berylliumsulfate from this contaminated mixture of crystals. The presentinvention may be used to solve this problem by leaching the mixedcrystals with cold water containing sufficient ammonium sulfate toproduce at the conclusion of the leaching step, a saturated solution ofberyllium sulfate containin preferably about 110 grams of ammoniumsulfate per liter. By this procedure, the ammonium alum will not bedissolved and the amount of calcium sulfate in the saturated berylliumsulfate would be at the minimum of 0.5 gram of calcium sulfate perliter. It will be understood that after separation of the alum crystalsand careful filtration of the mother liquor, the filtrate wouldcorrespond to a solution identified by the point a. It consequently maybe evaporated and beryllium sulfate crystallized from it in the samemanner as the solution described previously in connection with line abc.

In connection with the discussion of the supersaturation effect, aprevious description followed a solution along the line ,fghi, and itwas pointed out then that by the time the crystallization of berylliumsulfate had been completed, the ammonium sulfate and calcium sulfateconcentrations had arrived at the point n. Since the latter point fallsin an insoluble field, reference was made to the influence of thesupersaturation effect. Now it will be apparent to those skilled in theart that it is unnecessary to proceed from the point m to the point nsince after the solution has been concentrated by evaporation to thepoint m, additional ammonium sulfate may be added to the concentratedsolution to shift the ammonium sulfate-calcium sulfate ratio to anyselected position within the curve, as to the position designated bypoint m. After such addition has been made, the solution may be cooledso as to crystallize beryllium sulfate from it. When the crystallizationhas been completed, the concentrations of ammonium sulfate and calciumsulfate will be designated by the point n. It will therefore be seenthat this treatment has avoided the penetration of the insoluble fieldhi, and hence has avoided the necessity of relying upon thesupersaturation effect to hold the calcium sulfate in solution. 7

From what has just been said, it should not be thought that thesupersaturation effect should be avoided wherever possible. On thecontrary, the effect adds materially to the utility of the invention,since by virtue of the supersaturation elfect it is possible to treatsolutions which could not be treated successfully otherwise, andfurthermore, all treatments are rendered less critical and therefore aremore easily and cheaply controlled. For example, if one determined inthe manner described above that a given amount of calcium sulfate wouldrequire a certain initial concentration of ammonium sulfate per literunder equilibrium conditions, he would be able actually to utilizeammonium sulfate concentrations which are either somewhat under thedetermined value or somewhat over it. Stated in another manner, thesupersaturating effect permits a given concentration of ammonium sulfateto care for not only corresponding equilibrium amounts of calciumsulfate but actually to care for somewhat greater amounts. In view ofthis condition it will be appreciated that the supersaturating elfect ishelpful in several ways. It avoids the necessity for adhering exactly toequilibrium conditions, and it permits greater amounts of calciumsulfate to be retained in solution than could be retained if strictequilibrium conditions prevailed. The supersaturating effect isespecially useful in solutions which contain approximately the maximumamount of calcium sulfate allowable under equilibrium conditions. Forexample, if it were found that after the solution had been concentratedand cooled to room temperature to crystallize beryllium sulfatetherefrom, the solution would contain around 3.3 grams of calciumsulfate per liter, one would be inclined to believe that the inventioncould not be used to hold this latter amount of calcium sulfate insolution during the last stages of the crystallization step. Inpracticing the invention, however, I have found that if the peakconcentration of ammonium sulfate of about 490 grams per liter isprovided, the above amount of calcium sulfate may be held in solution.Under such conditions, however, the calcium sulfate is not retained insolution permanently, but only for a limited period of time; that is,the solution ultimately breaks down and some of its calcium sulfatecontent crystallizes out. It will be understood from what has been saidpreviously, that the breakdown is the result of the unstable nature ofthe supersaturated solution and that the period of time required toinduce the breakdown depends largely upon the instability or degree ofsupersaturation. In practicing the invention, however, I have found thatthe breakdown may be delayed for from one to eight hours aftercrystallization of beryllium sulfate has been commenced. It will beappreciated that if the solution breaks down within one hour, thesupersaturating effect is usually of little value. If, however, thebreakdown is delayed for a period of, say, three or four hours, then theentire crystal lization of beryllium sulfate may be completed, and thecalcium-free crystals of beryllium sulfate may be removed from contactwith the solution, before the breakdown occurs. Under such conditions,the supersaturating effect is beneficial since it permits the inventionto be employed in treating solutions which could not be treatedsuccessfully under equilibrium conditions. The supersaturation effect,therefore, is of considerable practical value.

The foregoing discussion has had to do with solutions saturated inberyllium sulfate and having not to exceed about 1.7 grams per liter ofcalcium sulfate in solution at room temperature. It is observed,however, that the invention is applicable to the treatment of saturatedberyllium sulfate solutions containing substantially more than 1.7 gramsper liter of calcium sulfate maintained in solution by the presence of asufiicient concentration of ammonium sulfate. Such solutions would notordinarily be encountered in applying the present invention toprocedures such as those of the Sawyer and Kjellgren patent which havebeen discussed above but they may otherwise be encountered and, asindicated, may be treated in accordance with the present invention toproduce substantially calcium-free beryllium sulfate. For example, if asolution saturated in beryllium sulfate at room temperature contained insolution 325 grams per liter of ammonium sulfate and contained 2.5 gramsof calcium sulfate, it is apparent from the drawing that such solutioncould be treated in accordance with the present invention to producecalcium-free beryllium sulfate by evaporation and cooling. By referenceto the drawing it will be observed that the stated concentrations ofammonium sul fate and calcium sulfate are in a ratio somewhat greaterthan 125 to 1 and that the ammonium sulfate would serve to hold thecalcium sulfate in solution so that, by concentrating the solution,beryllium sulfate would be crystallized for separation free of calciumcontamination.

In the practice of the present invention when solutions of berylliumsulfate contain calcium sulfate in a solid state as well as in solution,it ordinarily will be desirable to remove at least the major part of thesolid calcium sulfate by filtration. However, it will be clear from thepreceding discussion that if for any reason it is undesirable orinconvenient to filter the beryllium sulfate solution the solid calciumsulfate can be solubilized prior to or during concentration of thesolution if a sufiicient amount of ammonium sulfate is added to thesolution to make the ratio of ammonium sulfate to calcium sulfate notless than 125 to 1.

Now that various applications of the invention have been described, itwill be understood to be subject to certain limitations as to ammoniumsulfate and calcium sulfate concentrations and ratios. For example, itwill be understood that the invention may be applied to a solution whichcontains up to about 3 grams per liter. While it was pointed outpreviously that as much as 3.3 grams of calcium sulfate per liter couldbe retained in solution, it will be understood that this concentrationwas measured in the cold solution after the crystallization had beencompleted. On the contrary, the limit of 3 grams per liter justmentioned is the limit as measured in the leaching solution after it hasbeen concentrated to the point where it is saturated in berylliumsulfate and is ready to be treated to crystallize beryllium sulfatetherefrom, Accordingly, some additional crystallization can be effectedbeyond this 3-gram limit before the supersaturation value of 3.3 gramsper liter is reached.

As shown by the curve, an ammonium sulfatecalcium sulfate ratio of 125to 1 is about the lowest practical limit for such ratio, while an upperlimit of about 800 to 1 is established by the limiting value of ammoniumsulfate. Thus as noted above, when the calcium sulfate concentration isabout one-half gram per liter, practical difliculties are encountered ifthe ammonium sulfate concentration is increased to much over 406 gramsper liter as measured in the concentrated solution prior tocrystallization. For general uses, ratios between about 1'75 and 400 to1 are preferred, while for treating a solution in accordance with thefirst procedure described herein, ratios between 290 and 250 to l arepreferred. It will also be understood that the invention does notcontemplate ammonium sulfate concentrations below about grams per liter.

Thus, in the working of the process the amount of free ammonium sulfateused will range from about 80 grams per liter upward with ratios ofammonium sulfate to calcium sulfate of to 1 or higher. As is shown bythe drawing, a ratio of ammonium sulfate to calcium sulfate of about1'75 to l or higher will maintain the calcium content in saturatedsolution while, as has been explained, ratios ranging from to 1 down to125 to 1 will maintain the calcium content in at least supersaturatedsolution permitting crystallization and separation of the berylliumsulfate substantially free of contamination with calcium.

The invention has been explained through examples and illustrationswhich have dealt largely with the essential features of the invention.It should be remembered, however, that numerous minor variations basedon the variations of solubility shown in the drawing may be made by oneskilled in the art in extending the application and utility of theinvention. Moreover, various minor factors introduce appreciablevariations. For example, the solubility of calcium sulfate in aberyllium sulfate solution containing any given concentration of freeammonium sulfate within the ranges set forth above, will, in general, beincreased as the beryllium sulfate concentration is decreased.Furthermore, the effect of temperature on the solubility of calciumsulfate in solutions of the character involved here should beconsidered, since, in general, more calcium sulfate maybe dissolved in aboiling sulfate solution than in one at, say 28 C. This increasedsolubility is indicated in the figure by the dash line A-A. Whererefrigeration is employed to effect crystallization at temperaturesbelow ambient room temperatures, due consideration should be given tothe reduced solubility induced by the use of such lower temperatures. Itis noted in this connection that the term fcrystallizationf unlessexpressly limited, is used herein in a broad sense including any of theknown procedures or treatments for effecting crystallization. It shouldalso be recognized that the solubility of calcium sulfate is affected byvarious ions which may be allowed to be present in the solution byreason of the fact that they do not interfere with the formation ofnearly pure crystals of beryllium sulfate. Since the maximum permissibleconcentrations depend on the material itself, and since the actualamounts of such ions may vary considerably depending on the type of orewhich has been treated, and on the character of the previous steps inthe process, their concentrations cannot be expressed readily.Nevertheless, the concentrations which may normally be present orencountered are capable of introducing sizable variations in thesolubility of calcium sulfate. Those skilled in the art will appreciatethat these various factors as just discussed permit numerous departuresto be made from the procedure and examples described above withoutdefeating the objects of the invention and without departing from itsprinciples.

It will be understood that the invention is not 14 exceed 3 grams perliter of solution and an amount of ammonium sulfate not less than about80 grams per liter of solution and such that the ratio of ammoniumsulfate to calcium sulfate is at least approximately 125 to 1;evaporating said solution to a point where a substantial amount ofberyllium sulfate is crystallized on cooling but where the resultantincrease in concentration of ammonium sulfate will still maintain thecalcium sulfate in at least supersaturated solution; and separating thecrystals of beryllium sulfate so formed.

BENGT R. F. KJELLGREN.

